For decades, paper fiber contaminated with inks, adhesives, dirt and other contaminants has been a persistent problem to the recycling industry. Impact inks are particularly difficult to remove, and result in brightness losses in recycled paper products. During recycling, tacky materials such as pressure-sensitive and non-pressure-sensitive adhesives and the like contribute to the formation of what are commonly referred to in the trade as "stickies." Stickies tend to deposit on the felts, wires, drier cans and other paper machine parts, and disrupt the smooth operation of paper machines. They also degrade the functional and cosmetic properties of paper products made with recycled paper fiber.
Although stickies are introduced into the wastepaper stream from many different sources, the predominant contributors are pressure-sensitive adhesives (PSAs), non-pressure-sensitive binding and packaging adhesives, paper coatings, waxes, and thermoplastic non-impact print toners. Stickies and other contaminants can be removed from paper pulp using physical and chemical means, but their inherent physical properties reduce the overall efficiency of these processes.
Printed wastepaper is deinked and recycled into pulp by a combination of pulping, screening, cleaning (forward and flow-through), floatation, washing and solids concentrating process steps. The sequence and redundancy of each step in the process varies at each mill and depends upon the nature of the incoming wastepaper stream and the product specifications of the outgoing pulp. The screening operation is the preferred operation for removing contaminants, because it enables the contaminants to be permanently removed from the pulp as solid materials early in the process. This simplifies subsequent processing of the pulp.
Unfortunately, the complete removal of PSA contaminants from paper fiber is complicated by several inherent properties common to all normally self-adhesive materials. Under conditions of shear, as are encountered in the pulping operation and during the transfer of pulp between deinking steps, conventional PSAs fragment or disperse into small to moderate size particles (about 100 to 5000 microns in diameter). Such particles can easily deform with heating and/or pressure, allowing the contaminants to pass through conventional screens and filtering equipment. Such deformation is quite prevalent under the high pressure drops and high temperatures (&gt;37.degree. C.) common to commercial deinking operations.
The smooth operation of subsequent washing and floatation operations is hindered by the presence of the fragmented or dispersed, tacky globules, where the globules are dispersed or fragmented PSA particles. It is believed that the globules are in the form of discrete particles after contact with water. The particles' inherent tackiness causes them to adhere to other contaminants and to paper fibers, and further complicates the recycling process. In addition, many PSA products have densities between 0.95 and 1.05 g/cm.sup.3, reducing the efficiency of removal with centrifugal cleaners.